Post-installation steps for Linux

Estimated reading time: 11 minutes

This section contains optional procedures for configuring Linux hosts to work better with Docker.

Manage Docker as a non-root user

The docker daemon binds to a Unix socket instead of a TCP port. By default that Unix socket is owned by the user root and other users can only access it using sudo. The docker daemon always runs as the root user.

If you don’t want to use sudo when you use the docker command, create a Unix group called docker and add users to it. When the docker daemon starts, it makes the ownership of the Unix socket read/writable by the docker group.

Warning: The docker group grants privileges equivalent to the root user. For details on how this impacts security in your system, see Docker Daemon Attack Surface.

To create the docker group and add your user:

  1. Create the docker group.

    $ sudo groupadd docker
    
  2. Add your user to the docker group.

    $ sudo usermod -aG docker $USER
    
  3. Log out and log back in so that your group membership is re-evaluated.

    If testing on a virtual machine, it may be necessary to restart the virtual machine for changes to take affect.

    On a desktop Linux environment such as X Windows, log out of your session completely and then log back in.

  4. Verify that you can run docker commands without sudo.

    $ docker run hello-world
    

    This command downloads a test image and runs it in a container. When the container runs, it prints an informational message and exits.

Configure Docker to start on boot

Most current Linux distributions (RHEL, CentOS, Fedora, Ubuntu 16.04 and higher) use systemd to manage which services start when the system boots. Ubuntu 14.10 and below use upstart.

systemd

$ sudo systemctl enable docker

To disable this behavior, use disable instead.

$ sudo systemctl disable docker

If you need to add an HTTP Proxy, set a different directory or partition for the Docker runtime files, or make other customizations, see customize your systemd Docker daemon options.

upstart

Docker is automatically configured to start on boot using upstart. To disable this behavior, use the following command:

$ echo manual | sudo tee /etc/init/docker.override

chkconfig

$ sudo chkconfig docker on

Use a different storage engine

For information about the different storage engines, see Storage drivers. The default storage engine and the list of supported storage engines depend on your host’s Linux distribution and available kernel drivers.

Troubleshooting

Kernel compatibility

Docker will not run correctly if your kernel is older than version 3.10 or if it is missing some modules. To check kernel compatibility, you can download and run the check-compatibility.sh script.

$ curl https://raw.githubusercontent.com/docker/docker/master/contrib/check-config.sh > check-config.sh

$ bash ./check-config.sh

The script will only work on Linux, not macOS.

Cannot connect to the Docker daemon

If you see an error such as the following, your Docker client may be configured to connect to a Docker daemon on a different host, and that host may not be reachable.

Cannot connect to the Docker daemon. Is 'docker daemon' running on this host?

To see which host your client is configured to connect to, check the value of the DOCKER_HOST variable in your environment.

$ env | grep DOCKER_HOST

If this command returns a value, the Docker client is set to connect to a Docker daemon running on that host. If it is unset, the Docker client is set to connect to the Docker daemon running on the local host. If it is set in error, use the following command to unset it:

$ unset DOCKER_HOST

You may need to edit your environment in files such as ~/.bashrc or ~/.profile to prevent the DOCKER_HOST variable from being set erroneously.

If DOCKER_HOST is set as intended, verify that the Docker daemon is running on the remote host and that a firewall or network outage is not preventing you from connecting.

IP forwarding problems

If you manually configure your network using systemd-network with systemd version 219 or higher, Docker containers may be unable to access your network. Beginning with systemd version 220, the forwarding setting for a given network (net.ipv4.conf.<interface>.forwarding) defaults to off. This setting prevents IP forwarding. It also conflicts with Docker’s behavior of enabling the net.ipv4.conf.all.forwarding setting within containers.

To work around this on RHEL, CentOS, or Fedora, edit the <interface>.network file in /usr/lib/systemd/network/ on your Docker host (ex: /usr/lib/systemd/network/80-container-host0.network) and add the following block within the [Network] section.

[Network]
...
IPForward=kernel
# OR
IPForward=true
...

This configuration allows IP forwarding from the container as expected.

DNS resolver found in resolv.conf and containers can't use it

Linux systems which use a GUI often have a network manager running, which uses a dnsmasq instance running on a loopback address such as 127.0.0.1 or 127.0.1.1 to cache DNS requests, and adds this entry to /etc/resolv.conf. The dnsmasq service speeds up DNS look-ups and also provides DHCP services. This configuration will not work within a Docker container which has its own network namespace, because the Docker container resolves loopback addresses such as 127.0.0.1 to itself, and it is very unlikely to be running a DNS server on its own loopback address.

If Docker detects that no DNS server referenced in /etc/resolv.conf is a fully functional DNS server, the following warning occurs and Docker uses the public DNS servers provided by Google at 8.8.8.8 and 8.8.4.4 for DNS resolution.

WARNING: Local (127.0.0.1) DNS resolver found in resolv.conf and containers
can't use it. Using default external servers : [8.8.8.8 8.8.4.4]

If you see this warning, first check to see if you use dnsmasq:

$ ps aux |grep dnsmasq

If your container needs to resolve hosts which are internal to your network, the public nameservers will not be adequate. You have two choices:

  • You can specify a DNS server for Docker to use, or
  • You can disable dnsmasq in NetworkManager. If you do this, NetworkManager will add your true DNS nameserver to /etc/resolv.conf, but you will lose the possible benefits of dnsmasq.

You only need to use one of these methods.

Specify DNS servers for Docker

The default location of the configuration file is /etc/docker/daemon.json. You can change the location of the configuration file using the --config-file daemon flag. The documentation below assumes the configuration file is located at /etc/docker/daemon.json.

  1. . Create or edit the Docker daemon configuration file, which defaults to /etc/docker/daemon.json file, which controls the Docker daemon configuration.

    bash $ sudo nano /etc/docker/daemon.json

  2. Add a dns key with one or more IP addresses as values. If the file has existing contents, you only need to add or edit the dns line.

    {
    	"dns": ["8.8.8.8", "8.8.4.4"]
    }
    

    If your internal DNS server cannot resolve public IP addresses, include at least one DNS server which can, so that you can connect to Docker Hub and so that your containers can resolve internet domain names.

    Save and close the file.

  3. Restart the Docker daemon.

    $ sudo service docker restart
    
  4. Verify that Docker can resolve external IP addresses by trying to pull an image:

    $ docker pull hello-world
    
  5. If necessary, verify that Docker containers can resolve an internal hostname by pinging it.

    $ docker run --rm -it alpine ping -c4 <my_internal_host>
    
    PING google.com (192.168.1.2): 56 data bytes
    64 bytes from 192.168.1.2: seq=0 ttl=41 time=7.597 ms
    64 bytes from 192.168.1.2: seq=1 ttl=41 time=7.635 ms
    64 bytes from 192.168.1.2: seq=2 ttl=41 time=7.660 ms
    64 bytes from 192.168.1.2: seq=3 ttl=41 time=7.677 ms
    

Disable dnsmasq

Ubuntu

If you prefer not to change the Docker daemon’s configuration to use a specific IP address, follow these instructions to disable dnsmasq in NetworkManager.

  1. Edit the /etc/NetworkManager/NetworkManager.conf file.

  2. Comment out the dns=dnsmasq line by adding a # character to the beginning of the line.

    # dns=dnsmasq
    

    Save and close the file.

  3. Restart both NetworkManager and Docker. As an alternative, you can reboot your system.

    $ sudo restart network-manager
    $ sudo restart docker
    
RHEL, CentOS, or Fedora

To disable dnsmasq on RHEL, CentOS, or Fedora:

  1. Disable the dnsmasq service:

    $ sudo service dnsmasq stop
    
    $ sudo systemctl disable dnsmasq
    
  2. Configure the DNS servers manually using the Red Hat documentation.

Allow access to the remote API through a firewall

If you run a firewall on the same host as you run Docker and you want to access the Docker Remote API from another host and remote access is enabled, you need to configure your firewall to allow incoming connections on the Docker port, which defaults to 2376 if TLS encrypted transport is enabled or 2375 otherwise.

Specific instructions for UFW

UFW (Uncomplicated Firewall) drops all forwarding traffic and all incoming traffic by default. If you want to access the Docker Remote API from another host and you have enabled remote access, you need to configure UFW to allow incoming connections on the Docker port, which defaults to 2376 if TLS encrypted transport is enabled or 2375 otherwise. By default, Docker runs without TLS enabled. If you do not use TLS, you are strongly discouraged from allowing access to the Docker Remote API from remote hosts, to prevent remote privilege-escalation attacks.

To configure UFW and allow incoming connections on the Docker port:

  1. Verify that UFW is enabled.

    $ sudo ufw status
    

    If ufw is not enabled, the remaining steps will not be helpful.

  2. Edit the UFW configuration file, which is usually /etc/default/ufw or /etc/sysconfig/ufw. Set the DEFAULT_FORWARD_POLICY policy to ACCEPT.

    DEFAULT_FORWARD_POLICY="ACCEPT"
    

    Save and close the file.

  3. If you need to enable access to the Docker Remote API from external hosts and understand the security implications (see the section before this procedure), then configure UFW to allow incoming connections on the Docker port, which is 2375 if you do not use TLS, and 2376 if you do.

    $ sudo ufw allow 2376/tcp
    
  4. Reload UFW.

    $ sudo ufw reload
    

Your kernel does not support cgroup swap limit capabilities

You may see messages similar to the following when working with an image:

WARNING: Your kernel does not support swap limit capabilities. Limitation discarded.

If you don’t need these capabilities, you can ignore the warning. You can enable these capabilities in your kernel by following these instructions. Memory and swap accounting incur an overhead of about 1% of the total available memory and a 10% overall performance degradation, even if Docker is not running.

  1. Log into Ubuntu as a user with sudo privileges.

  2. Edit the /etc/default/grub file.

  3. Add or edit the GRUB_CMDLINE_LINUX line to add the following two key-value pairs:

    GRUB_CMDLINE_LINUX="cgroup_enable=memory swapaccount=1"
    

    Save and close the file.

  4. Update GRUB.

    $ sudo update-grub
    

    If your GRUB configuration file has incorrect syntax, an error will occur. In this case, repeat steps 3 and 4.

  5. Reboot your system. Memory and swap accounting are enabled and the warning does not occur.

Next steps

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